When an RF pulse is heard as a click on a radio or picked up by a lightning flash detector one does not know the location of the source or even if the signal came from lightning rather than a man-made RF source. On the other hand, when one sees lightning under or in a cloud there is no question regarding occurrence or location. The problem with visual lightning detection is that while the human eye is an excellent detector at night it is very poor in daylight. During the day, cloud to ground (CG) lightning can be seen against a dark background, but it is difficult to see against a bright background. Moreover, intracloud (IC) flashes, which are the best indicators of thunderstorm development and occur 5 to 50 times as frequently as CG flashes, are almost never seen in daylight.
Because CG lightning flashes are easily observed and therefore commonly thought of as representing "lightning", it is generally not realized that the rate of IC lightning is much higher than CG lightning. Not only does IC lightning provide a better statistical basis for detection and quantifying thunderstorm activity, it usually commences in the upper portion of clouds during the updraft phase of convective cell development--well before electrification of the lower part of the cloud by the subsequent downdraft which produces CG discharges. Experience indicates that optical signals from IC lightning were usually detected 10 to 15 minutes before any CG lightning is reported. Also, CG flashes are relatively "few and far between", and not well correlated with the most active cloud volumes, whereas IC lightning is frequent and closely related to convective regions. This is significant because other aspects of thunderstorms besides lightning, such as hail and microburst winds, are hazardous to certain ground operations. Thus the ability to detect IC lightning is not only advantageous for monitoring thunderstorm development and intensity, but it can provide earlier warning than systems which only detect CG lightning.
Strobe lights, such as found on aircraft, also generate pulses of light which are optically detectable. In the past, several detectors have been developed to indicate the presence of strobe lights to provide a warning indication to pilots that another aircraft was in the vicinity. Although a pilot is generally interested in both the presence and direction of both other aircraft and thunderstorms, no device capable of discriminating between flashes produced by aircraft strobes and flashes produced by thunderstorms has been developed.